term used to show up those who call you a "smut muffin" thinking you're too blonde too figure out that its an insult. tums is simply smut backwards, and cupcake is just a way of mimicking the "smut muffin" phrase and one uping it by adding frosting. "tums cupcakes" are generally people too sheltered or stuck up to realize how obvious the insult is, and are usually the aggressor of the situation, calling their victims smut muffin freely not aware of the consequences.
ass: "youre just a smut muffin."
smartie: "I know thats an insult you tums cupcake. just because my hair is blonde doesnt mean im not smart enough to see through your dumb insults. read a dictionary."
ass: "why are we talking about tums? does your stomach hurt?"
smartie: "wouldnt you like to know."
A word a bunch of smut muffins came up with to insult someone really cool. Nobody is really sure of the exact definition, but it seems to be used to express some sort of sexual interest.
Smut muffin: "hey bro, you're a tums cupcake!"
Cool person: "oh... Ok" *continues eating lunch*